On its website the group said: "We are going to tell you how Olympic medals are won. We hacked World Anti-Doping Agency databases and we were shocked with what we saw.

"We will start with the US team which has disgraced its name by tainted victories. We will also disclose exclusive information about other national Olympic teams later."

Biles takes medication for Attention Deficit Hyperactivity Disorder, while Serena Williams is alleged to have been granted permission to take drugs commonly used to treat muscle injuries.

WADA has confirmed the attacks, claiming they were an attempt to undermine the anti-doping body.

"WADA deeply regrets this situation and is very conscious of the threat that it represents to athletes whose confidential information has been divulged through this criminal act," director general Olivier Niggli said.

"We are reaching out to stakeholders, such as the IOC, IFs (international federations) and NADOs (national anti-doping organisations), regarding the specific athletes impacted.

"WADA condemns these ongoing cyber-attacks that are being carried out in an attempt to undermine WADA and the global anti-doping system.

"WADA has been informed by law enforcement authorities that these attacks are originating out of Russia.

"Let it be known that these criminal acts are greatly compromising the effort by the global anti-doping community to re-establish trust in Russia further to the outcomes of the agency's independent McLaren investigation report."

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